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Since 2013, several commercial solutions have arisen to partially overcome these issues. But to avoid undesired artifacts and non-reproducibility, it is important to understand the physical phenomena involved when dealing with syringe pumps in microfluidics. (Moreover, always be sure that your preferred commercial contact (even at Elveflow) completely understands your scientific experiment before choosing your flow control setup. For a similar tutorial about flow oscillation when using syringe pumps in microfluidics, click here.

When changing flow rates using a syringe pump within a microfluidic system, the settling time it takes to reach 95% of the desired flow rate can range from hundredths of a millisecond to several hours. The main reason for this is that microfluidics often deals with high fluidic capacitance (elasticity of the fluidic system, compliance of tubes) and small flow rates.

How and why does the responsivness vary depending on the setup?

When the motor of the syringe pump pushes the syringe’s piston, the pressure increases in the fluidic system. To reach the desired flow rate within the chip, physics rules say that you have to set the pressure of chip inlet at the right value. The pressure required to reach the desired flow rate increases linearly with the fluidic resistance of the chip and increases linearly with the settled flow rate.

In a perfect world where everything is non-deformable, this pressure increase should be reached instantaneously (say at the speed of sound in your setup). But fluidic systems have compliance since they deform with pressure (acting like an RC filter connected to a current source in electronics). When the system deforms, it slows down the pressure increase and this slows down the establishment of the right flow rate in the chip.

Thus, the flow rate settling time increases linearly with:

- The fluidic resistance (i.e. pressure)

- The fluidic capacitance of your system

(To come back to an electronics analogy, the current source (the syringe pump) will take time to charge the capacitance at the desired voltage (pressure) and will be then limited by the electrical resistance of the system).

The settling time is more or less independent of the flow rate. But at low flow rates, another artifact can appear: if the syringe volume is too high and the precision of the motor step too low, the syringe motor will just … not move … for … some time …

Another common problem that occurs at low flow rates is that if the mechanical contact between the piston and the motor is not perfect, as often, the motor will initially push nothing or cause jerking because of dry friction.

Microfluidic flow switch in hundredths of a millisecond

How to fasten flow change in your microfluidic chip?

Bubbles:

Air bubbles are one of the biggest enemies of microfluidicists. The presence of air bubbles in your microfluidic system or in your syringe will act as a big capacitance and will dramatically increase your response time. This is a very common problem in microfluidics – always take care to avoid bubbles if you want to get reproducible and exploitable scientific results when using a syringe pump. You can find more information about how to avoid air bubbles in microfluidic devices in this tutorial.

If you need a bubble trap kit to remove bubble from you capillary, click here

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Dust or living cell:

If dust or cells settle on or cork your microchannel during your experiment, the fluidic resistance will increase and your response time too. This situation often happens during cell biology applications on a microfluidic chip. In the worst situation, if the fluidic resistance becomes too high, you will encounter a second issue – the pressure in your device will dramatically increase, leading to chip or syringe leakage and damage. One way to avoid this is to filter your sample and take care when dealing with cells in microfluidics. If you are dealing with cells, you can also use a flow meter with your syringe pump, which will enable you to check automatically or manually if your channel is beginning to cork. Or, if it continues to be a big issue, you can eventually stop using syringe pumps and use a microfluidic pressure controller to overcome the issue. For more information about our brand of microfluidic pressure controller : click here

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Elasticity of the tubing and syringe:

Reducing the fluidic compliance by using glass syringes, PEEK, PTFE or glass capillaries instead of TYGON will increase the responsiveness of your flow control. Unfortunately, it will also increase flow oscillations, the second drawback of syringe pumps. You can find main type of microfluidic tubing here.

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Elasticity of the chip:

In some cases, reducing the fluidic compliance by using a glass microchip will also increase the responsiveness of your flow control. Unfortunately, it will also increase flow oscillations, the second drawback of syringe pumps.

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Mechanical compliance in the contact between motor and syringe piston:

Using a high precision microfluidic syringe pump should help you to partially overcome this issue. Of course, it is important to always screw in your syringe tightly. You will find here a tutorial about microfluidic pulseless syringe pump.

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Volume of the syringe & Precision of the motorstep:

As mentioned before, it is important to always take a look at the adequation between flow rate, syringe volume and minimal motor step. Here too, using a high precision microfluidic syringe pump will help.

If you want to enhance the reactivity of your microfluidic syringe pump you can also click here to discover our syringe Pump Fast Response Kit. The components of our Syringe Pump Fast Response Kit can easily be added to your setup to quickly and efficiently forward your flow rate instructions.

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Other alternatives to overcome responsiveness issues of syringe pumps

Aside from the basic optimization previously described, there are also different commercial solutions to overcome responsiveness issues in the use of syringe pumps in microfluidics. All these solutions have advantages and drawbacks; none of them are perfect. When choosing your setup, always take a careful look at the datasheets (particularly the fine print and small asterisks) and check that the representative really understands your experiment before choosing your setup.

For more tutorial about microfluidics, please visit our other tutorials here: «Microfluidics tutorials». The photos in this article come from the Elveflow® data bank, Wikipedia or elsewhere if precised. Article written by Guilhem Velvé Casquillas and Timothée Houssin and revised by Lauren Durieux.